I agree with Bob about the weight. Usually, I add a mass object that gets the weight right and then move it around until the predicted CG matches the real CG. My impression is that the weight error is not so much in the accuracy of the database as in the things we leave out of the simulation, like the glue and paint. I've been helping a TARC team and their rocket came out several ounces heavy. I then considered the amount of 5-minute epoxy that had been consumed and there was a definite corellation with the under estimate of weight.
Based on that experience, I'll throw out two other factors that will impact the RS accuracy, temperature and CD. After having accounted for the actual weight, RS said they needed an E28 to reach 750'. Not even close. The biggest engine I had in my box was an F39 reloadable and that only got to 634'. The default temperature for RS is 59 deg F. We were flying in temperatures in the upper 30's. They put in actual temperature and barometric pressure and still predicted a higher altitude that was being obtained, but not by much. Adjusting CD upwards a bit the actual altitudes from several flights were matched reasonably well. The rocket was not painted but had several coats of sanding sealer and had been sanded smooth, so we felt that calling the finish "smooth" in RS was reasonable.
Back to the drawing board. They cut out 4" of overall length from the first design and built a second prototype, being careful of how much glue was being used. The resulting rocket was about 3 oz lighter. This one also got painted before flight. In similar temperatures, it is going much higher. In fact, with enough ballast to fly at what was the empty weight of the original rocket, it flew to 796 feet.
What was learned from this? With lower air temperatures, the air is denser. RS accounts for this and computes higher drag, hence lower altitude, but you have to put in the right temperature. This suggests that the delay that works in warm conditions may be long for colder conditions. Also, in the denser air, the drag on the parachute is also higher, so it will hang longer. It appears that they need to ballast to one weight and use one size chute to qualify here in Connecticut and will probably need more ballast and a larger chute if they make the flyoffs in Virginia.